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Why do some chimps throw rocks at trees?

Chimpanzees are our closest evolutionary cousins, but if you see one with a stone in its hand you might want to keep your distance. Recent research demonstrates that the bonds of kinship will not keep a chimp from piling up stones and hurling them at zoo visitors if they get too close. A new study of wild chimps at four sites in West Africa now shows that they also like to throw stones at trees. A team of several dozen researchers reports online today in Scientific Reports that it has observed chimps routinely throwing and banging rocks at arboreal targets, resulting in the kind of stone accumulations pictured above. The data come from 14 to 17 months of observing chimp populations in Guinea-Bissau, Guinea, Liberia, and Ivory Coast. According to the team, this is the first time such behavior has been detected. Why do the chimps do it? The researchers still don’t know, but they’ve put forward two main hypotheses. One possibility is that the behavior is a way for males to make noise that attracts females and scares off other males, similar to displays like tree-root drumming. This kind of ritualized behavior is typical of all chimp populations. Another idea is that the accumulation of stones might have a symbolic or cultural meaning, for example marking territory in the way that humans do with stone cairns. Indeed, the team points out, some indigenous people in West Africa build stone shrines at trees they consider sacred. Until they figure it out, however, Science recommends that all visitors to chimp territory keep well out of the firing line.